The present invention relates to an electronic power supply device for driving a discharge lamp, comprising a switching regulator formed by a chopper transistor driven with a variable cyclic ratio through a driver stage, a smoothing inductance, a freewheel diode and a smoothing capacitor.
Such power supply devices are particularly adapted to control the brightness of discharge lamps to be dosed, and more especially of the so-called "daylight lamps" which equip stage lighting projectors. In order to offer a good reproduction of colors, these lamps generally receive a compound filling based on argon, mercury, halogens and rare earths. The operation of the lighting, such as bringing the lamp up to temperature, maintaining the lamp under established operating conditions and relighting the lamp after extinction require special precautions on the part of the power supply manufacturer because of the transitory phenomena which appears in the solid, liquid or gaseous substances of the filling on the one hand and because of excessive transient voltages or currents which affect the electric or electronic components on the other hand.
During the cold lighting phase, which lasts from half a minute to three minutes depending on the type and the rated power, the voltages required for driving the lamp increase considerably. An ordinary regulated power supply is not suitable, even if it is adapted to regulate either a current, a voltage, or an electric power. The aim of the invention is therefor to program the successive phases of bringing the lamp into service in accordance with a sequence which takes into account the requirements related to safety, reliability and the rapidity of temperature rise, without the life expectancy and the dosability of the light which it emits being altered.
To this end, the electronic power supply device of the present invention comprises means for lowering the switching frequency of the copper transistor during the heating-up period of the lamp until its striking voltage has reached about 70% of its rated value.
With this arrangement, the conduction period .tau. of the chopper transistor may be increased beyond the time for establishing the current in said transistor, while increasing the DC component of the current delivered, which allows the rate of temperature rise of the lamp to be increased.
In a particular embodiment of the invention, said means for lowering the frequency comprise an analog gate sensitive to the striking voltage acting on the time constant of an oscillator driving the driver stage through a cyclic ratio modulator.
According to another characteristic of the invention, the power supply device comprises an electronic cut-out switch, responsive to the current delivered by the chopper transistor, so as to shunt the driving of the driver stage when this current exceeds a maximum predetermined value.
This cut-out switch will for example be formed by a threshold device controlling the conduction of a transistor connected in parallel across the input of the driver stage.
The action of such a cut-out switch is equivalent to shortening the conduction time .tau. of the chopper transistor, thus causing the growth of the instantaneous current to be stopped.
Generally, a resistor is inserted upstream of the regulator, so as to limit the surge current during the switching on of the power supply device. According to yet another characteristic of the invention, this resistor is connected in parallel with a thyristor or triac whose gate electrode is continuously supplied from a secondary winding wound on the smoothing inductance, through a rectifier, a smoothing capacitor and a resistive adapter.
Thus, the resistance for limiting the surge current is automatically shunted as soon as the chopper transistor begins to operate.